1. Field of the Invention
The invention relates to cameras, and more particularly to auto focus function of cameras.
2. Description of the Related Art
A lens-depth-of-field (DOF) represents a distance range in which objects of an image have a higher clarity than a threshold clarity. When a camera system generates an image with a specific focal length, objects with a distance range from the camera system are clear in the image, and other objects outside of the distance range are blurry in the image. Referring to FIG. 1A, a schematic diagram of a lens-depth-of-field of a camera system 100 is shown. The camera system 100 photographs objects 101, 102, and 103 according to a specific focal length. Assume that the specific focal length corresponds to a distance between the object 102 and the camera system 100. The object 102 is therefore clear in an image generated by the camera system 100. When a distance between an object and the camera system 100 falls to the distance range between distances D1 and D2, the object has a higher clarity than a clarity threshold of the image. The distance range between distances D1 and D2 is referred to as a lens-depth-of-field (DOF).
Because the distance between the object 101 and the camera system 100 is less than the distance D1, the object 101 in the image has a lower clarity than the clarity threshold and is blurred. Similarly, because the distance between the object 103 and the camera system 100 is greater than the distance D2, the object 103 in the image has a lower clarity than the clarity threshold and is blurred. Thus, when the camera 100 wants to photograph clear images of the objects 101 and 103, the focal length of the camera system 100 is needed to be adjusted.
Extending les-depth-of-filed (EDOF) is a new technology which uses image processing techniques to improve the clarity of objects in an image and extend the DOF of the image. Referring to FIG. 1B, a schematic diagram of an extending lens-depth-of-field of a camera system 150 is shown. The camera system 150 photographs the objects 161, 162, and 163 according to a specific focal length to obtain an image, and then uses an EDOF process to improve the clarity of the objects 161, 162, and 163 in the image. Assume that the specific focal length corresponds to a distance between the object 162 and the camera system 150. The object 162 is therefore clear in the image generated by the camera system 150. The image is processed by the EDOF process, the depth-of-field of the image is extended from the distance range D1˜D2 shown in FIG. 1A to the distance range D1′˜D2′. Because the locations of the objects 161 and 163 fall in the distance range D1′˜D2′, then the clarity of the objects 161 and 162 in the processed image is also higher than that of the clarity threshold.
An auto focus technology can automatically adjust a focal length of a camera system to improve clarity of an image. A conventional camera system with an auto focus function requires a long time period for adjusting the focal length, delaying photographing of images and leading to inconvenience for users. If the EDOF technology is used in a camera system to shorten the time period required by the auto focus function in adjustment of the focal length, the performance of the camera system can be improved. A camera system which combines the benefits of the auto focus function and the EDOF technology is therefore required.